You can start by reading the main page which is rendered from Markdown. In the "History" tab you can see all the changes that were made to create the current article version. Each change consists of a diff, along with information about the author and the change date. This is similar to a git log. You can discover more articles with the "List Articles" menu option on the left side. By default all known articles from different instances are shown (remote articles are indicated by @domain). You can interact with remote articles just the same as local articles. It is also possible to list local articles only.
To continue, register a new account and you are logged in immediately without further confirmation. If you are admin of a newly created instance, login to the automatically created admin account instead. Login details are specified in the config file (by default user ibis and password also ibis).
On a new instance only the default Main Page will be shown. Use "Create Article" to create a new one. You have to enter the title, text and a summary of the edit. Afterwards press the submit button, and you are redirected to the new article. You can also make changes to existing articles with the "Edit" button at the top. The article text uses standard markdown. Additionally you can link to other articles with [[Title@example.com]]. If multiple users attempt to edit an article at the same time, Ibis will attempt to merge the changes automatically. If this is unsuccessful, the user has to perform a manual merge (again like in git). For remote articles, there is additionally a "Fork" option under the "Actions" tab. This allows copying a remote article including the full change history to the local instance. It can be useful if the original instance is dead, or if there are disagreements how the article should be written.
To kickstart federation, paste the domain of a remote instance into the search field, eg https://example.com. This will fetch the instance data over Activitypub, and also fetch all articles to make them available locally. The search page will show a link to the instance details page. Here you can follow the instance, so that new articles and edits are automatically federated to your local instance. You can also fetch individual articles from remote instances by pasting the URL into the search field.